1. Field of the Invention
The present invention relates to an image bearing member and an image forming method, an image forming apparatus and a process cartridge using the image bearing member.
2. Discussion of the Background
Recently, organic photoconductors (image bearing members) have been used in place of inorganic photoconductors for a photocopier, a facsimile machine, a laser printer and a multifunctional device thereof in light of performances and advantages, such as, optical characteristics, for example, a wide range of optical absorption wavelength and a large amount of absorption of light; electric characteristics, for example, high sensitivity and stable chargeability; a wide selection of materials; ease of manufacturing; inexpensiveness; and toxic-free property.
In addition, demand for the size reduction of an image forming apparatus accelerates the size reduction of an image bearing member. Also, high speed performance and maintenance-free performance have been demanded. Therefore, an image bearing member having high durability has been desired. From this point of view, an organic photoconductor is easy to wear in general because the surface layer thereof is mainly made of a low molecular weight charge transport material and an inert polymer. When such an organic photoconductor is repetitively used in the electrophotography process, the organic photoconductor tends to be abraded under mechanical stress by a developing system or a cleaning system. In addition, in accordance with the size reduction of toner particles for improving the quality of images, the rubber of a cleaning blade is hardened and the contact pressure between an image bearing member and a cleaning blade is increased to improve the cleaning property. This accelerates the abrasion of an image bearing member. Such abrasion of an image bearing member causes deterioration of electric characteristics, for example, the sensitivity and the chargeability, resulting in abnormal images, such as, deterioration of image density and the background fouling. When an image bearing member is locally damaged by abrasion, the damaged portion causes streaks on an image resulting from bad cleaning performance on the image bearing member. Currently, this abrasion or damage is a controlling factor of the lifetime of an image bearing member and once an image bearing member has such abrasion or damage, the image bearing member must be replaced immediately to sustain image quality and performance.
It is desired to reduce the amount of abrasion described above to obtain an organic photoconductor having a high durability. This is an imminent issue to be solved in this field.
As a technology to improve the anti-abrasion property of an image bearing member, for example, (1): unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637 describes a technology in which a curing binder is used for a surface layer; (2): JOP S64-1728 describes a technology in which a polymer charge transport material is used; and (3) JOP H4-281461 describes a technology in which an inorganic filler is dispersed in a surface layer. Among these technologies, with regard to the curing binder of (1), the residual voltage tends to rise due to bad compatibility between the curing binder and a charge transport material and remaining impurities, for example, a polymerization initiator or non-reacted groups, which results in reduction in image density. The anti-abrasion property of an organic photoconductor using the polymer charge transport material of (2) or the dispersed inorganic filler of (3) is improved in some degree but has not reached the level desired for an organic photoconductor.
Furthermore, to improve the anti-abrasion property and anti-damage property of the organic photoconductor described in (1), Japanese Patent No. (hereinafter referred to as JP) 3262488 describes an organic photoconductor containing an acrylate monomer cured compound having multiple functional groups. Although there is a description that the surface layer provided on the photosensitive layer contains the acrylate monomer cured compound, there is no specific description about a charge transport material but just a description that a charge transport material can be contained in the surface layer. In addition, when a charge transport material having a low molecular weight is simply contained, a problem of the compatibility between the cured compound and the charge transport material arises. This problem tends to cause precipitation of a transport material having a low molecular weight and white turbidity phenomenon. Also, the mechanical strength of the organic photoconductor easily deteriorates.
Furthermore, this organic photoconductor is manufactured by reacting the monomer in the state in which a polymer binder is contained so that the curing reaction is not sufficiently conducted. In addition, the compatibility between the cured material and the binder resin is bad and therefore, the phase separation tends to occur during the curing reaction and lead to formation of a rough surface, resulting in bad cleaning performance.
As the anti-abrasion technology for a photosensitive layer in place of these technologies, for example, JP 3194392 describes a charge transport layer manufactured by using a liquid application formed by a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin contains a binder resin having a carbon-carbon double bond and a binder resin having no carbon-carbon double bond. That is, the binder resin having a carbon-carbon double bond reacts with the charge transport material but the binder resin having no carbon-carbon double bond does not react with the charge transport material. It is notable that this organic photoconductor has an anti-abrasion property and electric characteristics in a good combination. However, there is a tendency that when the binder non-reactive with the charge transport material is used, the compatibility between the binder resin and the cured material obtained by the reaction between the monomer mentioned above and the charge transport material is bad and therefore, phase separation tends to occur during cross-linking and leads to formation of a rough surface, which results in bad cleaning performance. In addition, as described above, the binder resin prevents curing of the monomer and since the monomers specified in the JP 3194392 have only two functional groups, the density of the cross-linking is not sufficient. Therefore, the anti-abrasion property obtained in this case is still insufficient.
In addition, even when the binder is reactive with the transport material, the number of the functional groups contained in the monomer and the binder resin is not sufficient. Therefore, it is difficult to have a good combination of the combined amount of the charge transport material and the cross-linking density and thus, the electric characteristics and anti-abrasion property are not sufficient.
For example, JOP 2000-66425 describes a photosensitive layer containing a compound cured from a positive hole transport material having at least two chain reaction polymerization functional groups.
However, this photosensitive layer contains the bulky positive hole transport material having at least two chain reaction polymerization functional groups so that the cured compound has distortion and thus the internal stress is strong. Therefore, the surface of the photosensitive layer tends to be rough and cracking easily occurs over time, meaning that the surface does not have a sufficient durability.
As an image bearing member (photoconductor) having a different charging system, for example, JOP 2001-166518 describes an image bearing member having a protective layer containing a compound cured from a positive hole transport material having at least two chain reaction polymerizable functional groups and electroconductive particulates.
However, this image bearing member is charged by infusion charging by a contact type charging device and the electric resistance of the surface layer decreases due to the electroconductive particulates which improves the mechanical strength of the surface layer. When a charger, for example, a commonly used corona charger or a contact type charging roller, is used, the electric resistance of the surface layer tends to fluctuate due to the environmental change (temperature and humidity), ozone emitted from the charger and attachment of NOx products on the surface layer, which easily causes production of abnormal images, for example, flown images.
In addition, for example, JP 3123733 describes an image bearing member containing particulates having a needle form with an aspect ratio of not less than 1:10 in the surface protective layer to improve the anti-abrasion property and the anti-damage property by a combination of (1) and (3). JOP H10-20536 also describes an image bearing member having a surface layer containing particulates having a needle form in polyurethane resins. However, the resins dispersed in these image bearing members do not have a function of charge transport. Therefore, the electric characteristics thereof are not sufficient in both image bearing members.
Similarly, for example, JOP 2005-99688 describes an image bearing member in which a cross-linking resin layer cured from at least a monomer having at least three radical polymerizable functional groups without a charge transport structure and a radical polymerizable compound having a charge transport structure serves as a surface layer and filler particulates are dispersed in the surface layer. This image bearing member is stable to environmental change and has good electric characteristics and mechanical durability. However, further improvement is demanded with regard to the durability.
In addition, full color electrophotographic apparatuses of late tend to adopt polymerized toner in terms of image quality and environment factors. As these polymerized toners become closer to spherical in shape, the sharpness of images is improved. However, such toners easily slip through the blade in the case in which a cleaning blade is adopted as a cleaning system for retrieving residual toner. This causes production of abnormal images having streaks.
The mechanism of toners close to a sphere (hereinafter referred to as ‘spherical toner’) slipping through a cleaning blade is considered to be that a spherical toner easily rotates between a cleaning blade and an image bearing member. Therefore, it is relatively difficult to remove such spherical toner particles in comparison with typical toner particles, which do not have a spherical form.
As a method of improving the cleaning property of spherical toner, for example, JOP 2005-107490 describes a method in which the surface energy of an image bearing member is reduced by adding silicone oil and/or fluorine particulates to a cross-linked resin layer functioning as a surface layer cured from at least a monomer having at least three radical polymerizable functional groups without a charge transport structure and a radical polymerizable compound having a charge transport structure. This image bearing member has a good combination on mechanical strength and cleaning property for a spherical toner. However, further improvement is demanded with regard to the durability and cleaning property for a spherical toner.